Active steering systems may continuously and intelligently alter the relationship between a driver's steering inputs at the steering wheel and the angle of the steered road wheels (i.e., the road wheel angle (RWA)) of the vehicle. An active steering system, for example, varies the degree that the road wheels turn in response to rotation of the steering wheel via a mechatronic auxiliary system. At lower speeds, this technology may reduce the amount that the steering wheel must be turned (improving performance in situations such as parking and other urban area traffic maneuvers), and at higher speeds, the system helps to prevent increased steering responsiveness resulting from increased speeds (providing improved direction stability).
Examples of active steering systems include, but are not limited to, a steer-by-wire system, in which there is no mechanical connection between the steering wheel and the steering of the road wheels, and a superimposed steering system, in which the steering angle that is generated at the steering wheel by the driver is superimposed on a steering angle generated by a servo motor, in a superimposition gear mechanism, and the sum of the two steering angles is transmitted to the steering of the road wheels. Such active steering systems provide various advantages, including, for example, the ability to change the angle of the steered road wheels without the intervention being directly perceptible to the driver.
At higher steering rates (e.g., when the driver of the vehicle is turning the steering wheel quickly), active steering systems may create a situation known as “steering catch-up,” in which the power steering cannot provide sufficient assist and the driver feels an increased torque at the steering wheel. Active Front Steering (AFS) systems, for example, may provide an electronically controlled superposition angle (i.e., an overlay angle) to the steering wheel angle (SWA) provided by the driver to increase the SWA (and the effective steering speed), thereby creating a higher occurrence of steering catch-up situations.
To remedy this problem, AFS systems have been developed, which include steering anti-catch-up (SAC) features that may reduce steering catch-up and high current draws, for example, during slow speed driving (e.g., during parking and rolling when the driver is most likely to turn the steering wheel quickly). A SAC system may, for example, limit the rate of the desired pinion angle (i.e., the output of the AFS variable gear ratio (VGR) feature or the desired road wheel angle (RWA)) at particular vehicle speeds. In other words, to avoid catch-up when the driver steers quickly while driving slowly, the SAC may reduce the demanded road wheel turning speed by calculating an offset that will be applied to the desired RWA (i.e., the output of the VGR). Thus, as shown, for example, in FIGS. 3A and 3B, with the SAC, when the driver steers fast, the driver will get less overlay angle then he or she would get without the SAC, where the angle would be purely based on the VRG feature. When the fast steering event is over, the SAC will then attempt to smoothly reduce and remove this offset without the driver noticing.
Current SAC systems, which are generally dependent on steering speeds to determine offset calculation and correction, are therefore limited to offset reduction during periods of slow steering (i.e., when the system detects that the driver is steering at a slow rate via a steering wheel velocity sensor). Accordingly, as illustrated in FIG. 3A, such systems may, therefore, fail to remove offsets that are present, for example, when the driver is steering very slowly (i.e., at a speed that cannot be detected by looking at the steering wheel velocity signal), and when the steering wheel is back in the center position (e.g., at times t2 and t4 of FIG. 3A, wherein the desired RWA is zero).
It may, therefore, be advantageous for an active steering system to include a SAC feature that can correct steering offsets during all driving situations. It may be further advantageous to provide a SAC system that can completely remove the steering offset when the steering wheel is turned back to the center position so that the vehicle will drive straight ahead when the steering wheel is in the center position.